Gas sensors are used to detect combustible gases and these sensors typically include an electrically-driven measuring element. This measuring element can be an infrared radiator with a detector, a semiconductor element or a catalytic measuring element.
During operation or in the case of a fault, a temperature increase can occur at these measuring elements which basically constitutes the danger of an ignition of the combustible gas mixture to be measured. For this reason, the measuring element is encapsulated in a housing which prevents a possible explosion in the interior of the housing from igniting the ambient surrounding the sensor.
Typically, these explosion-tight housings are of metal. The access of the gas is ensured via a porous material, for example, a wire fabric or metal sinter. This material functions as a flame barrier. To provide an electrical contact of the measuring element from the outside, an opening is provided in the housing through which the cable or metal pins are passed and the opening is closed thereafter with a suitable casting material. Conventional casting materials are, for example, epoxy resin or cement. Typical disadvantages of cast cable passthroughs are inadequate tightness because of separation of the cured cast mass from the housing and a minimum structural size which results from standardization requirements for the thickness of casting material. Thus, and in accordance with present-day standards, the minimum casting thickness is 3 mm for a housing having an interior volume of less than 10 cm and 6 mm for housings less than 100 cm3.
Pressure-tight encapsulated sensors are also known which comprise a metal housing which is embedded in plastic material. Specific minimum casting thicknesses must be maintained even for these housing configurations and complex and costly proof tests must be carried out with respect to maintaining specific standard requirements. In addition, sensor housings with plastic components are not suitable for high temperature applications and for the use in atmospheres containing solvents or acids. A sensor having a plastic housing is described, for example, in international patent publication WO 2004/048955 A2.
A catalytically active gas sensor having a glass passthrough is disclosed in European patent application EP 0 094 863 A1. The catalytically active sensor element is disposed in a sensor housing which is delimited by a porous gas-permeable sinter material. Two metal pins, which contact the sensor element, are passed to the outside through a glass disc at the lower side of the sensor housing. The sensor element is surrounded by zeolite material in order to reduce the energy consumption and increase service life via the insulating action and absorption characteristics of the zeolite material.
The glass passthrough for the metal pins disclosed in European patent application EP 0 094 863 A1 is, however, not suitable for the use in pressure-tight, encapsulated, explosion-protected sensor housings. Sensor housings of this kind must be so dimensioned that they withstand a pressure from 1.5 to four times which can build up in the interior of the sensor housing in the case of an explosion. In addition, it must also be ensured that the housings can withstand explosions in the interior of component assemblies to which the sensor is attached (for example, gas measuring apparatus, electrical terminal boxes, et cetera). With the explosions, pressures of more than several hundred bar can occur.
A flat disc of glass material (that is, a glass disc having a low ratio of thickness to diameter which is, in addition, weakened because of the integration of several metal pins as is the case in known sensor arrangements) will be destroyed by the formation of fissures when subjected to the pressure load occurring in a gas explosion.